The usual reinforcing or strengthening means used in doors for motor vehicles consists of horizontal supports or beams, attached to the vertical door struts. The protective effect of such reinforcements is limited to a certain height of the colliding body. Doors of this design are heavy, since the forces are conducted to the vertical struts of the doorway, which must be built in a very sturdy manner. Therefore, a door of this sort is costly to construct. The involvement of the door sill as well as the rest of the car body when the door is deformed in a collision is achieved only to a very inadequate extent.